Cuboid pressure vessels are conventionally known. In such a cuboid pressure vessel, stress concentration occurs at many sites thereof, compared with a spherical or cylindrical pressure vessel. It is desirable to reduce the occurrence of this stress concentration as much as possible. Therefore, reinforcement ribs are frequently formed in the cuboid pressure vessel. Further, pressure vessels are generally required to be reduced in weight as much as possible while satisfying necessary design requirements therefor.
In relation to this problem, it is disclosed in Patent Document 1 that reinforcement ribs are provided in a cuboid pressure vessel in order to suppress the occurrence of stress concentration thereto, and a weight reduction of the pressure vessel is attained by devising the configuration of the reinforcement ribs. Concretely, a pressure vessel according to Patent Document 1 is provided with a square-shaped top wall, a square-shaped bottom wall, and an external wall which connects the top wall to the bottom wall. The top wall is provided with a square-shaped top-wall main body, and reinforcement ribs provided in an upright manner on the outer surface of the top-wall main body. The external wall has an opening. The opening is occluded by a lid which is configured so that the opening can be occluded. The reinforcement ribs are formed in an area which contains a central section of the top-wall main body and which excludes four corner sections of the top-wall main body. Concretely, the reinforcement ribs include: a square-shaped outer peripheral rib formed so as to linearly interconnect the centers of respective adjacent sides of the top-wall main body; lattice-shaped ribs provided in an upright manner within a central section of an area surrounded by the outer peripheral rib; and connecting ribs formed respectively so as to linearly interconnect the opposed vertexes of the outer peripheral rib.
Such a pressure vessel can be used also as, for example, a gas cooler or a container capable of housing a heat exchanger or the like therein. When this pressure vessel is used as the gas cooler, it is generally used with the surface provided with the reinforcement ribs facing downward. This is based on the reason that the bottom wall side of the pressure vessel needs to be reinforced while the top wall side is sufficiently reinforced. Concretely, piping connections for connecting external piping to the heat exchanger housed in the pressure vessel are generally formed in a plurality of portions of the top wall. The top wall side is sufficiently reinforced with these piping connections. On the other hand, since the piping connections are not provided on the bottom wall side, the reinforcement of the bottom wall side is often needed.
In the pressure vessel of Patent Document 1, the configuration of the reinforcement ribs is devised as described above, whereby a weight reduction from reduced material use is attained while satisfying necessary design requirements for the pressure vessel. However, the pressure vessel of Patent Document 1 cannot be said to be sufficiently reduced in weight.